1. Field of the Invention
The invention relates to a spindle unit used in a machine tool.
2. Discussion of Background
For example, Japanese Patent Application Publication No. 6-8005 describes a spindle unit in which an intermediate housing is interposed between a front bearing arranged at the front side (tool side) of a main spindle and a front housing, and the intermediate housing is held by a hydrostatic bearing with a damping property, so that chatter vibration of the main spindle during machining is suppressed. Japanese Patent Application Publication No. 2004-106091 describes a spindle unit in which a hydrostatic air bearing is provided next to a ball bearing arranged at the front side (tool side) of a main spindle, so that chatter vibration of the main spindle during machining is suppressed. US 2008/0231129 A1 describes a spindle unit in which a plurality of radial magnetic bearings and a plurality of radial displacement sensors are arranged at the front side (tool side) of a main spindle, and electromagnets of the radial magnetic bearings are controlled on the basis of the radial displacements detected by the radial displacement sensors, so that resonance of the main spindle is suppressed.
However, the conventional arts have a problem that the damping force is weak, and therefore chatter vibration of the main spindle is not suppressed sufficiently.